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A new study claims our Moon could have been born from a blob of the Earth's mantle, blasted outwards by the explosion of a runaway nuclear georeactor.

If correct, it would replace the scientifically accepted model for the origins of the Moon, known as the 'giant impact theory'.

That theory suggests a Mars-sized planet called Theia, slammed into the early proto-Earth about 62 million years after the solar system's birth - 4.6 billion years ago.

The iron and nickel core of both planets coalesced to form the Earth, while the lighter silicate mantle and crust, ejected into space forming the Moon - explaining why the Moon isn't as dense as Earth.

Computer simulations suggest about 80% of the Moon came from the impactor and 20% from the proto-Earth.

"Both Moon and Earth rocks have almost the same isotopic content, which is put down to debris mixing in orbit as the Moon formed," they write in a paper published on the electronic preprint website arXiv.

"While that works for lighter elements, it can't easily account for the identical ratio of heavier elements such as chromium, neodymium and tungsten."

Blasted out

According to the paper, the Earth was once a rapidly spinning mass of molten rock; the force of gravity only just preventing it from spinning apart.

"Just a slight kick would have would have been enough to eject a blob into orbit, eventually forming the Moon," they write.

The idea's been around for ages, but rejected because nobody could work out where the energy to kick a lunar-sized blob into orbit, could have come from.

de Meijer and van Westrenen believe the energy could have come from a nuclear explosion deep in the Earth's mantle.

According to the researchers, high concentrations of radioactive elements such as uranium and thorium at a region known as the 'core-mantle boundary', could have caused a runaway nuclear chain reaction going supercritical. This would have resulted in an explosion of material blasted into orbit eventually forming the Moon.

"Our hypothesis provides an alternative explanation for the striking similarity in elemental and isotopic composition of the Earth's mantle and lunar rocks," they write.

"It's based around the disputed hypothesis of a naturally occurring georeactor going super critical."

O'Toole says, "the assumptions required for their idea to work are so large compared to the generally accepted theory, that if you apply Ockham's Razor and ask what's most likely, well, this isn't it!"

"The idea of the Earth spewing out the Moon is great for science fiction, but a bit hairy for science fact."